The present invention relates to dicalcium phosphate compositions having improved monofluorophosphate compatibility, and to a process for the preparation thereof.
Dicalcium phosphate dihydrate has been used as a dental polishing agent in toothpastes and powders for many years.
This material is typically produced by first reacting a slaked lime slurry with phosphoric acid to form a dicalcium phosphate dihydrate precipitate, and then separating the dicalcium phosphate dihydrate precipitate from the mother liquor, after which it is dried and milled to form the final product as a fine powder.
One serious problem which was initially encountered in the use of dicalcium phosphate dihydrate in toothpaste was the tendency of the dicalcium phosphate to "set-up" and become lumpy. When this occurs in toothpaste formulations, it makes it difficult to extrude the toothpaste from the tube in which it is usually packaged.
A second problem was encountered with the advent of the use of monofluorophosphate additives in toothpaste formulations. It was found that the monofluorophosphate components would react with the dicalcium phosphate whereby the monofluorophosphate component was converted from a water-soluble form to an insoluble form. Since the beneficial effect of monofluorophosphate additives in toothpaste are understood to be derived principally from the water-soluble form, it has become important to develop toothpaste formulations which permit an effective amount of monofluorophosphate component to remain in the water-soluble state.
The term "monofluorophosphate-compatibility" has been used as a term-of-art to describe the tendency of such formulations to permit the monofluorophosphate component to remain in the water-soluble state.
The monofluorophosphate compatibility of a particular formulation may be determined by a variety of methods. Preferably, the monofluorophosphate compatibility of a formulation is determined by actually preparing the formulation, placing it in storage for a predetermined period of time under controlled conditions, and then determining the amount of water-soluble monofluorophosphate which remains in the formulation after having been stored under these conditions. Alternatively, a simulated formulation, such as the dicalcium phosphate dihydrate to be tested, glycerine and a known amount of a monofluorophosphate component, such as sodium monofluorophosphate can be "quick aged" by maintaining it at an elevated temperature for one or more hours, and then determining the amount of water-soluble monofluorophosphate remaining after such conditioning. There are, of course, many other methods for measuring the relative monofluorophosphate compatibility of various samples of dicalcium phosphate dihydrate.
U.S. Pat. No. 2,287,699 teaches that dicalcium phosphate dihydrate may be stabilized by adding a small amount of an alkali metal pyrophosphate to the mother liquor, at a controlled pH, during the preparation of the dicalcium phosphate. Specifically, it is taught that after precipitation of the dicalcium phosphate in the mother liquor, a small amount of alkali metal pyrophosphate should be added and the entire slurry then heated for a short period of time, while maintaining the pH of the mother liquor above 7.
Alternatively, the precipitate may be treated during the subsequent washing step.
It is also known to those skilled in the art that other forms of pyrophosphate can also be used to stabilize the dicalcium phosphate.
Another method for stabilizing dicalcium phosphate is disclosed in U.S. Pat. 2,018,410. This patent teaches that dicalcium phosphate can be stabilized by the addition thereto of a magnesium salt such as trimagnesium phosphate, magnesium sulfate, magnesium stearate, or dimagnesium phosphate.
Copending U.S. patent application Ser. No. 106,637 now U.S. Pat. No. 4,312,843 teaches a method for preparing dicalcium phosphate dihydrate compositions having improved monofluorophosphate compatibility which involves the addition of pyrophosphoric acid to the reaction mixture and termination of the reaction within a very limited pH range of from about 4.9 to about 5.5.
Surprisingly and unexpectedly in view of the teachings of the prior art, it has now been found that improved monofluorophosphate compatibility can be achieved with the addition of pyrophosphoric acid and termination of the reaction at pHs below about 4.9.